1. Technical Field
This invention relates to a system and method for supplying and melting a particulate material comprising a powdered thermoplastic resin and for propelling the melted material onto the surface of a substrate to provide the substrate with a coating having desired physical properties and characteristics. More particularly, this invention relates to a flame spray coating system comprising a flame spray gun and means for supplying powdered particulate material, an oxidizing gas and a combustible gas to the gun; and a method for using the flame spray gun and the supply means to coat a substrate.
2. Prior Art
Methods and apparatus for flame spraying thermoplastic resins onto substrates are previously known, having been disclosed for example in U.S. Pat. Nos. 4,632,309 and 4,934,595.
In a spray gun of the type disclosed in U.S. Pat. No. 632,309, an open-atmosphere powdered flame spray gun and a method are disclosed in which a powderized thermoplastic material, combustion air and a combustion gas are delivered through a plurality of passageways extending through the spray gun body into an open mixing and combustion chamber defined by a cylindrical hood extending beyond the spray gun body. The resultant mixture is ignited and the thermoplastic material is melted in the flame combustion area while entrained in a stream of pressurized air that propels the melted material onto the substrate.
One limitation experienced through use of the method and apparatus disclosed in U.S. 4,632,309 was that projecting the stream of combustible gas into the combustion chamber at an oblique angle toward the axis of the combustion chamber and toward the central stream of propelling air and entrained particulate material actually caused a "pinching" of the stream of particulate material and limited the quantity of thermoplastic material that could be melted and delivered for spray coating. In addition, the angular delivery of the combustible gas in the combustion chamber was found to limit the size of the flame "tunnel" emanating from the combustion chamber, and therefore was a self-limiting factor in the total quantity of particulate thermoplastic material that could be melted for spray application. Further, if increased flow rates of particulate material were desired to be delivered by the spray gun, an improved hopper and eductor feed means were necessary to entrain and mix the desired quantity of particulate material in the stream of propelling air.
U.S. Pat. No. 4,934,595 disclosed a flame spray coating system comprising an improved flame spray gun, eductor mechanism and means for controlling the flow of the powdered thermoplastic material. One modification to the flame spray gun of U.S. Pat. No. 4,934,595 over the gun disclosed in U.S. Pat. No. 4,632,309 was the addition of a flexible diaphragm between the body member of the gun and the flame hood assembly. The purpose of the diaphragm was to function as a seal between the body member and flame hood assembly, and to better balance the flow of combustible gas (preferably propane) around the annulus supplying the combustible gas to the combustion chamber. Another modification was redirection of the longitudinal axes of the outermost array of circumferentially spaced combustion gas orifices so as to be parallel to the axis of the hood section and thereby alleviate "pinching" of the annular flow of propelling air and the circular stream of particulate material and conveying air emerging from the gun. Another modification was the provision of sections at the outlet ends of the flow nozzle and flow nozzle bore to promote radial expansion of the streams of conveying air (containing entrained powdered thermoplastic material) and propelling air, respectively, as they emerged from the gun.
The flame spray coating system disclosed in U.S. Pat. No. 4,934,595 was also adapted to control the flow of powder through the gun by means of a valve disposed in the gun handle that was manually operated to block the flow of conveying air and powder at the gun whenever the operator needed to stop coating. Such needs can frequently arise during a coating operation, for example, Whenever the operator reaches a discontinuity in the surface being coated, needs to change position, take a break, or the like.
Although the devices previously disclosed have proved to be effective for applying protective polymeric coatings to many different kinds of substrates, including for example, bridges, ship hulls, plant piping, and the like, disadvantages have been experienced that can limit their effectiveness under some conditions of use. One disadvantage encountered through use of the flame spray coating system disclosed in U.S. 4,934,595 is that blocking the flow of conveying air and entrained powder at the gun traps conveying air and entrained powder in the flow line from the eductor to the gun, permitting the powder to settle out. Whenever the flow is reestablished by opening the valve, the powder that settled in the flow line can be discharged from the gun in large "blobs" that are undesirable for evenly coating a substrate, or can collect inside fittings, chambers or orifices in the gun to impede flow and necessitate frequent disassembly and maintenance.
Another disadvantage encountered through use of the flame spray coating systems disclosed in the prior art is that different sized guns are required in order to achieve the different coverage areas and coating rates needed for various coating jobs. For example, a two inch diameter gun might be desirable for coating small diameter piping or other substrates having relatively small surfaces, whereas a four inch diameter gun might be desirable for coating larger surface areas on the same job. With the prior art devices, two separate guns would be required, with the necessity of completely shutting down the system to disconnect all flow lines from one gun and reconnect them to another. This would entail disconnecting and reconnecting the conveying air and powder supply line, the propelling air supply line and the combustible gas supply line.
A third disadvantage encountered through use of the prior art flame spray coating systems is that most of the thermoplastic powder is deposited on the surface of the substrate in a circular pattern in which the interior of the circle is substantially void. While improvements to the flame spray gun (flaring the diameter of the central bore and the inside diameter of the powder flow nozzle in the section adjacent to the outwardly facing plate surface of the flame hood assembly) disclosed in U.S. Pat. No. 4,934,595 aided in expanding the diameter of the powder pattern deposited on a substrate from a given distance, a flame spray gun is needed can that apply powder in an expanded, filled circular pattern.